One type of continuously variable transmission for continually varying a speed change ratio of a vehicle is a V-belt type transmission disclosed for example in Tokkai Hei 8-338515 published by the Japanese Patent Office in 1996.
The V-belt is looped around a drive pulley and driven pulley respectively comprising a groove, and the contact radius between the belt and a pulley is made to vary by changing the groove width of the pulleys so as to vary the rotation ratio of these pulleys.
The groove width of each pulley changes according to an oil pressure supplied to an oil chamber of each pulley. A line pressure supplied from an oil pump driven by an engine is constantly supplied to the oil chamber of the driven pulley, and an oil pressure which is the line pressure decompressed via an oil pressure control valve is supplied to the drive pulley. The oil pressure control valve is a spool valve, and the pressure of the oil chamber of the primary pulley is controlled by connecting an oil pump and oil tank selectively to the oil chamber of the drive pulley according to a position change of the spool.
The spool is connected to a step motor and drive pulley via a link, the spool position being made to vary according to a rotation position of the step motor. At the same time, the variation of drive pulley groove width is fed back to the spool position via the link. As a result, the spool stops in a position where a command value represented by the rotation position of the step motor and a real speed change ratio represented by the pulley groove width are balanced.
A controller controlling the speed change ratio is so designed that a desirable speed change response is obtained at a predetermined line pressure based on dynamic characteristics of the transmission.
However, the line pressure is often set according to a value representing the engine drive torque, e.g. the throttle opening. This is due to the reason that, If the line pressure is always kept constant, the load on the engine becomes large, and fuel consumption increases.
When the line pressure changes, the dynamic characteristics of the speed change mechanism also change, and the desired speed change is no longer obtained. As a result, it takes a long time until a target speed change ratio set by the controller coincides with the real speed change ratio.